Raman Study of 1T'-WS₂

Transition metal dichalcogenides (TMDs) have been researched for years and the influence of many parameters, such as composition, morphology, thickness, has been carefully analyzed. It is only in recent years that the phase has also been added as a crucial parameter that determines the properties of these materials, with the unusual phases of the TMDs exhibiting enhanced properties in applications such as catalysis and devices.

Our group has successfully synthesized a library of pure 1T’-MX₂ (M = Mo, W; X = S, Se) and their alloys, enabling us to study the intrinsic properties of these pure materials.

Raman spectroscopy makes an ideal candidate for investigating the phase of TMDs, due to its rapidity, non-invasiveness, and the small amount of sample required. Moreover, due to the metallic (or semi-metallic) character of the unusual phase TMDs, there is no issue with fluorescence interference, which is a common artifact when using Raman spectroscopy.

In this study, we unequivocally assign the Raman peaks to the specific molecular vibrations of 1T’-WS₂. After measuring the high-resolution Raman spectra of different 1T’-WS₂ crystals, we performed a statistical analysis to determine the exact position of the Raman peaks by fitting the spectrum with a convolution of six Lorentzian peaks. The residual analysis also enabled us to uncover a seventh peak, which was not observed at first since it is highly convoluted with another, more intense, peak. We concluded that the 1T’-WS₂ Raman spectrum displays six main peaks at 112.92 cm^-1 (1), 179 cm^-1 (2), 240.7 cm^-1 (3), 269.45 cm^-1 (4), 317.15 cm^-1 (5) and 407.92 cm^-1 (6), and an additional peak at approx. 125 cm^-1 (1’), highly convoluted with (1).

Angle polarized Raman spectroscopy in both parallel and perpendicular configuration complemented by theoretical considerations based on the single-crystal X-ray diffraction were used to assign the modes to the previously determined peaks.

The perpendicular-APR study proved that peaks (4), (5), and (6) belong to the same vibration mode, while peak (1) corresponds to the other one, while the parallel-APR study showed that peaks (2), (3), (4), (5), (6) represent the same vibration mode, which was attributed to A_g mode based on the symmetry of the APR curves. As a result, we were able to assign the modes to all peaks, one of them (1) corresponding to the B_g mode, while the other six corresponding to the A_g mode.

We chose 1T'-WS₂ due to its better stability compared to the other TMDs, but this study can be extended to the other 1T'-TMDs that our group works with (MoS₂, MoSe₂, WSe₂), as well as their alloys (MS_xSe_2-x, M = Mo, W). In the future, using alloys with a finely controlled composition, there is the possibility to develop an analysis method purely based on Raman spectroscopy to allow the quantitative characterization of the alloy in terms of its composition.